I have a couple of suggestions:
1. Something that could be a cool design element throughout the website is to use hand renderings of proposed landscape layouts. It's possible they could even serve as a template on the site itself (that text went over).
2. Get suppliers to provide photos. If it's a big chain or nation-wide supplier, they'll have pictures.
3. Take pictures of what you have and continually build up your stock (as supplies come in or are used) so that eventually you have enough stock to display on the website. I think it's particularly important to focus on specific products. For instance, I think it's useful to have pictures of a range of different mulch. If you ask me "Joe, do you want the shredded or the split mulch?" I"m going to go "huh?" So if there's a picture on the website (a close-up of the 7 different types of mulch) in a well-lit setting (so no shadow, no harsh light, correct white balance) then I can accurately choose. I think the same with stones (next to a ruler or some object of scale like a coin). The point is: you may not have pictures of 90% of the items which will bug you as a website designer. But identify which products are most critical to have visuals of so people can compare against competing products and chose wisely. I think stones and mulch are two examples.